Polysilsesquioxane has both inorganic and organic structures. The inorganic structure comprises at least one siloxane bond, and imparts the characteristics of an inorganic material, such as excellent transparency, heat resistance, hardness, insulation performance, etc. The organic structure comprises an organic group that is directly bonded to a silicon atom, and supplements the characteristics that are often insufficient in substances having only inorganic structures, such as solubility and dispersion stability in organic materials, adjusting the refractive index and the dielectric constant of polysilsesquioxane, imparting polysilsesquioxane with photo-curable properties, etc. In recent years, polysilsesquioxanes have been attracting public attention as organic/inorganic hybrid materials having the characteristics of both inorganic and organic materials.
The characteristics of polysilsesquioxane as an inorganic material and as an organic material become more notable when the polysilsesquioxane is made into a polymer having a higher molecular weight. However, this lowers its solubility in an organic material and results in difficulties in handling at the time of use. Generally, forming a substance into a polymer having a higher molecular weight and retaining excellent solubility are barely compatible. Therefore, an organic-solvent dispersion of fine polysilsesquioxane particles that attains both characteristics by forming a polysilsesquioxane into macromolecular fine particles and stably dispersing the resultant particles in an organic solvent has been demanded.
In recent years, production of highly transparent organic-solvent dispersions of fine polysilsesquioxane particles having not only both characteristics of inorganic and organic materials and excellent dispersibility in organic materials but also an average particle diameter of, for example, not greater than 40 nm, which is less than the wavelength of visible light has been demanded in fields wherein transparency is required, such as anti-reflection films, transparent adhesives, clear coating materials, optical waveguides, optical electronics materials, and photoprinted circuit board materials.
An aqueous dispersion of fine polysilsesquioxane particles has been conventionally known. For example, Patent Document 1 discloses that a colloidal polysilsesquioxane suspension having a particle size within the range of 10 to 1000 Å can be obtained by adding a trialkoxysilane to an aqueous solution containing a surfactant. However, this colloidal suspension is barely mixable and dispersible in an organic substance because its dispersion medium is water. Therefore, such a suspension is hardly usable if fine polysilsesquioxane particles have to be satisfactorily mixed or reacted with other organic substances. Therefore, in order to make fine polysilsesquioxane particles usable in various fields, production of an organic-solvent dispersion of fine polysilsesquioxane particles wherein an organic solvent is used as a dispersion medium instead of water has been awaited.
An organic-solvent dispersion of fine silica sol particles has also been conventionally known. Such dispersion can be obtained by, for example, concentrating an acidic aqueous silica sol that is prepared using sodium silicate as a starting material, adding isopropanol or like alcohol to obtain a dispersion of silica sol in an organic-solvents mixture, subjecting the dispersion to silylation by adding a silane coupling agent, and replacing its solvent with a methylethyl ketone or like hydrophobic solvent (for example, see Patent Document 2). Such dispersions can also be obtained by subjecting an alkaline aqueous silica sol formed from sodium silicate to deionization using ion exchange resin, concentrating the deionized silica sol by ultrafiltration, and blowing isopropanol steam thereinto while heating (for example, see Patent Document 3).
Patent Document 4 discloses a method for producing a highly pure hydrophilic organic-solvent dispersion of silica sol comprising the following steps (a) to (d).
(a) a first step of hydrolyzing an alkoxysilane in an aqueous alcohol solution;
(b) a second step of concentrating the silica sol solution obtained by hydrolysis;
(c) a third step of adjusting the silica sol solution to coupling by adding a silane coupling agent and adjusting its pH to neutral; and
(d) a fourth step of substituting the water in a silica sol solution with a hydrophilic organic solvent.
However, because the above organic-solvent dispersion of silica sol fine particles is obtained by subjecting silica sol to a surface treatment using a silane-coupling agent, the silica sol fine particles thereof mainly comprise siloxane bonds having an inorganic structure (i.e., free from an organic structure). Accordingly, the characteristics of organic- and inorganic-hybrid materials cannot be expected.
Patent Document 1:    Japanese Examined Patent Publication No. S52-12219
Patent Document 2:    Japanese Unexamined Patent Publication No. H11-43319
Patent Document 3:    Japanese Unexamined Patent Publication No. H8-169709
Patent Document 4:    Japanese Unexamined Patent Publication No. 2004-91220